1. Field of Invention
The present invention relates to a lens. More particularly, the present invention relates to a fixed-focus lens.
2. Description of Related Art
With the progress of modern video technology, image projection devices such as digital light processing (DLP) projectors and liquid crystal on silicon (LCOS) projectors are widely applied. One of the core elements in the projectors is a projection lens, which is used to project images on screens. Therefore, the quality of images is closely related to the optical quality of the projection lens. In the highly competitive market, manufacturers are all devoted to improving the quality of projection lenses and reducing the manufacturing cost, so as to improve the competitive edge of the image projection devices.
Referring to FIG. 1, a conventional projection lens 100 comprises a first lens group 110, a second lens group 120, and a third lens group 130 arranged in sequence, wherein the third lens group 130 is near a light valve 50. The first lens group 110 comprises four lenses 112, the second lens group 120 comprises one lens 122, and the third lens group comprises six lenses 132.
As the conventional projection lens 100 uses a relatively large number of lenses, the manufacturing cost is high. In addition, the projection lens 100 has a large length as many lenses are used. Therefore, a rear projection television (RPTV) using the projection lens 100 is normally thick. If the thickness of the RPTV is reduced, the aberration during optical imaging becomes serious and even problems such as ghost images occurs, which cause the degradation of the imaging quality.